If you want a really good finished, go get a can of primer first. Specifically paint designed for plastics. I use Krylon Fusion For Plastic
in flast white, grey, or black for doing plastic and fiberglass priming for model aircraft parts. Though it is claimed "no sanding necessary", be prepared to put a good amount of elbow grease into it. I don't go as crazy as some. I generally only go to about 600 grit as a final sanding.
Once you've primed the parts, automotive paint will give you the best finish. Rattle can spray paint work great if you are doing a single solid color, but the paint particles are much larger coming out of a spray can. Multiple colors with masked areas can give a pretty noticable seam. Most automotive shops can mix up and pre-thin paint for use in a Preval Spray Gun
. As others have implicitly pointed out, it can be quite expensive. My worst so far is $45 for a pint of paint, but I was trying to color match a metallic red. You won't be able to do air brush type work with a Preval sprayer though. You are still looking at masks and stencils.
One final thing to remember. When mixing paint types, do a test piece first and be patient. Allow the first layer to cure completely (perhaps days) before overlaying a layer from a different paint manufacturer. It will help you avoid orange peel and other problems.
*edit -- adding pictures*
The red on the cowl was color matched by Sherwin-Williams Automotive and sprayed with a Preval sprayer.
Here is what can happen when you get impatient with different paints.